共查询到20条相似文献，搜索用时 93 毫秒
1.
Field static corrections in general need be applied to all onshore seismic reflection data to eliminate the disturbing effects a weathering layer or near-surface low velocity zone has on the continuity of deep seismic reflections. The traveltimes of waves refracted at the bottom of the low velocity zone (or intermediate refracting interfaces) can often be observed as first breaks on shot records and used to develop a laterally inhomogeneous velocity model for this layer, from which the field static corrections can then be obtained. A simple method is described for computing accurate field statics from first breaks. It is based on a linearization principal for traveltimes and leads to the algorithms that are widely and successfully applied within the framework of seismic tomography. We refine an initial model for the low velocity layer (estimated by a standard traveltime inversion technique) by minimizing the errors between the observed first arrivals on field records and those computed by ray theory through an initial model of the low velocity layer. Thus, one can include more lateral velocity variations within the low velocity layers, which are important to obtain good field static corrections. Traditional first break traveltime inversion methods cannot, in general, provide such refined velocity values. The technique is successfully applied to seismic data from the Amazon Basin. It is based on a simple model for the low velocity layer that consists of an undulating earth surface and one planar horizontal refractor overlain by a laterally changing velocity field. 相似文献
2.
Derecke Palmer 《Geophysical Prospecting》2006,54(5):589-604
The performance of refraction inversion methods that employ the principle of refraction migration, whereby traveltimes are laterally migrated by the offset distance (which is the horizontal separation between the point of refraction and the point of detection on the surface), can be adversely affected by very near‐surface inhomogeneities. Even inhomogeneities at single receivers can limit the lateral resolution of detailed seismic velocities in the refractor. The generalized reciprocal method ‘statics’ smoothing method (GRM SSM) is a smoothing rather than a deterministic method for correcting very near‐surface inhomogeneities of limited lateral extent. It is based on the observation that there are only relatively minor differences in the time‐depths to the target refractor computed for a range of XY distances, which is the separation between the reverse and forward traveltimes used to compute the time‐depth. However, any traveltime anomalies, which originate in the near‐surface, migrate laterally with increasing XY distance. Therefore, an average of the time‐depths over a range of XY values preserves the architecture of the refractor, but significantly minimizes the traveltime anomalies originating in the near‐surface. The GRM statics smoothing corrections are obtained by subtracting the average time‐depth values from those computed with a zero XY value. In turn, the corrections are subtracted from the traveltimes, and the GRM algorithms are then re‐applied to the corrected data. Although a single application is generally adequate for most sets of field data, model studies have indicated that several applications of the GRM SSM can be required with severe topographic features, such as escarpments. In addition, very near‐surface inhomogeneities produce anomalous head‐wave amplitudes. An analogous process, using geometric means, can largely correct amplitude anomalies. Furthermore, the coincidence of traveltime and amplitude anomalies indicates that variations in the near‐surface geology, rather than variations in the coupling of the receivers, are a more likely source of the anomalies. The application of the GRM SSM, together with the averaging of the refractor velocity analysis function over a range of XY values, significantly minimizes the generation of artefacts, and facilitates the computation of detailed seismic velocities in the refractor at each receiver. These detailed seismic velocities, together with the GRM SSM‐corrected amplitude products, can facilitate the computation of the ratio of the density in the bedrock to that in the weathered layer. The accuracy of the computed density ratio improves where lateral variations in the seismic velocities in the weathered layer are known. 相似文献
3.
Improving modelling and inversion in refraction seismics with a first-order Eikonal solver 总被引：1，自引：0，他引：1
Isabelle Lecomte Håvar Gjøystdal ers Dahle & Ole Christian Pedersen 《Geophysical Prospecting》2000,48(3):437-454
A first-order Eikonal solver is applied to modelling and inversion in refraction seismics. The method calculates the traveltime of the fastest wave at any point of a regular grid, including head waves as used in refraction. The efficiency, robustness and flexibility of the method give a very powerful modelling tool to find both traveltimes and raypaths. Comparisons with finite-difference data show the validity of the results. Any arbitrarily complex model can be studied, including the exact topography of the surface, thus avoiding static corrections. Later arrivals are also obtained by applying high-slowness masks over the high-velocity zones. Such an efficient modelling tool may be used interactively to invert for the model, but a better method is to apply the refractor-imaging principle of Hagedoorn to obtain the refractors from the picked traveltime curves. The application of this principle has already been tried successfully by previous authors, but they used a less well-adapted Eikonal solver. Some of their traveltimes were not correct in the presence of strong velocity variations, and the refractor-imaging principle was restricted to receiver lines along a plane surface. With the first-order Eikonal solver chosen, any topography of the receiving surface can be considered and there is no restriction on the velocity contrast. Based on synthetic examples, the Hagedoorn principle appears to be robust even in the case of first arrivals associated with waves diving under the refractor. The velocities below the refractor can also be easily estimated, parallel to the imaging process. In this way, the model can be built up successively layer by layer, the refractor-imaging and velocity-mapping processes being performed for each identified refractor at a time. The inverted model could then be used in tomographic inversions because the calculated traveltimes are very close to the observed traveltimes and the raypaths are available. 相似文献
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三维复杂地形近地表速度估算及地震层析静校正 总被引：18，自引：6，他引：18
在地表一致性模型的基础上提出一种可适用于宽线剖面、弯曲测线、传统的二维和目前广泛使用的三维地震观测.在地形及近地表低降速带地质结构复杂的探区，低降速带厚度及速度估算的精度是静校正处理的关键.本研究根据三维地震观测的初至走时数据，利用最小平方与QR分解相结合的算法，在三维空间重建近地表低降速带速度模型，根据重建速度模型实现了静校正长波长分量与短波长分量的同步计算.分析了复杂的近地表低降速带模型初至波的性质，在观测值的自动拾取以及理论值的计算中充分考虑了可能成为初至波的直达波、折射波和反射波的利用，提高了低降速带速度模型反演的精度.在初至走时观测数据的拾取中，本研究采用分形算法克服了初至波波形差异以及折射波相位反转导致的拾取误差，实现了三维初至拾取的大规模全自动化运算.在射线路径与初至波理论走时的计算中，本研究采用一种计算量与模型复杂程度无关的三维射线追踪方法，该方法以最小走时射线路径保证了与观测数据有同等意义的初至波的射线追踪及理论走时的计算.野外实际资料的处理结果表明了方法的有效性. 相似文献
6.
最短路径射线追踪算法,用预先设置的网络节点的连线表示地震波传播路径,当网络节点稀疏时,获得的射线路径呈之字形,计算的走时比实际走时系统偏大. 本文在波前扩展和反向确定射线路径的过程中,在每个矩形单元内,通过对某边界上的已知走时节点的走时进行线性插值,并利用Fermat原理即时求出从该边界到达其他边界节点的最小走时及其子震源位置和射线路径,发展了相应的动态网络算法. 从而克服了最短路径射线追踪算法的缺陷,大大提高了最小走时和射线路径的计算精度. 相似文献
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Determination of a shallow velocity–depth model from seismic refraction data by coherence inversion1
Seismic refractions have different applications in seismic prospecting. The travel- times of refracted waves can be observed as first breaks on shot records and used for field static calculation. A new method for constructing a near-surface model from refraction events is described. It does not require event picking on prestack records and is not based on any approximation of arrival times. It consists of the maximization of the semblance coherence measure computed using shot gathers in a time window along refraction traveltimes. Time curves are generated by ray tracing through the model. The initial model for the inversion was constructed by the intercept-time method. Apparent velocities and intercept times were taken from a refraction stacked section. Such a section can be obtained by appling linea moveout corrections to common-shot records. The technique is tested successfully on synthetic and real data. An important application of the proposed method for solving the statics problem is demonstrated. 相似文献
8.
Time horizons can be depth-migrated when interval velocities are known; on the other hand, the velocity distribution can be found when traveltimes and NMO velocities at zero offset are known (wavefront curvatures; Shah 1973). Using these concepts, exact recursive inversion formulae for the calculation of interval velocities are given. The assumption of rectilinear raypath propagation within each layer is made; interval velocities and curvatures of the interfaces between layers can be found if traveltimes together with their gradients and curvatures and very precise VNMO velocities at zero offset are known. However, the available stacking velocity is a numerical quantity which has no direct physical significance; its deviation from zero offset NMO velocity is examined in terms of horizon curvatures, cable length and lateral velocity inhomogeneities. A method has been derived to estimate the geological depth model by searching, iteratively, for the best solution that minimizes the difference between stacking velocities from the real data and from the structural model. Results show the limits and capabilities of the approach; perhaps, owing to the low resolution of conventional velocity analyses, a simplified version of the given formulae would be more robust. 相似文献
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川东北复杂山地由于地表高程变化大、风化层厚度不均匀、低降速带速度横向变化大、高速层底界不稳定等因素,得到的地震资料具有严重的静校正问题,因此,如何解决山地静校正问题是后续地震资料处理的关键之一,本文首先分析高程静校正、折射静校正、层析静校正的基本原理和适用条件,结合实际复杂山地三维地震资料的特点,进行试验对比,提出了进行山地静校正的基本思路,即：首先进行高程静校正,这样可以利用高程静校正更容易拾取初至时间,然后利用折射静校正结合微测井等资料建立近地表速度-深度模型,以此速度-深度模型作为层析静校正的初始模型进行迭代处理,最后得到最终的近地表速度-深度模型和静校正值.根据以上处理流程,我们建立了适合于川东北山地三维复杂地表地震资料的静校处理正方法,并在实际生产过程中取得了良好的效果. 相似文献
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The widespread use of common depth point techniques has emphasized the need for accurate static corrections. Manual interpretation methods can give excellent results, but a computer technique is desirable because of the great volumn of data recorded in common depth point shooting. The redundancy inherent in common depth point data may be used to compute a statistical estimate of the static corrections. The corrections are assumed to be time-invarient, surface-consistent, and independent of frequency. Surface consistency implies that all traces from a particular shot will receive the same shot static correction and all traces from a particular receiver position will receive the same receiver correction. Time shifts are computed for all input traces using crosscorrelation functions between common depth point traces. The time shift for each trace is composed of a shot static, a receiver static, residual normal moveout if present, and noise. Estimates of the shot and receiver static corrections are obtained by averaging different sets of the measured time shifts. Time shifts which are greatly in error are detected and removed from the computations. The method is useful for data which has a moderate to good signal to noise ratio. Residual normal moveout should be corrected before estimating the statics. The program estimates the statics for correctly stacking common depth point traces but it is not sensitive to constant or very slowly changing static errors. 相似文献
11.
地震射线追踪方法技术在地震学领域有着较为广泛的应用,然而大多数算法建立在直角坐标系或球坐标系下,实际地球并非完美的球体,而是两极略扁的椭球体,因此,球坐标系下计算结果与真实情况存在一定误差.传统的做法一般是在球坐标系下进行计算,而后进行椭球校正.本文提出了一种直接在椭球体模型中采用分区多步最短路径算法进行多震相地震射线追踪的方法技术,实现了椭球坐标系下多震相地震波射线路径追踪和走时计算.与解析解的对比表明:该算法具有较高的计算精度,适用于任意形状的椭球体,且不需要进行额外的走时校正.数值模拟结果表明,计算所得P波和PcP反射波的走时与AK135走时表的误差小于0.1 s.当震中距较大时,使用球对称模型和椭球体模型计算所得的走时差异显著,说明采用椭球坐标系的必要性. 相似文献
12.
Ralph Bridle Nikolai Barsoukov Mohammad Al-Homaili Robert Ley Ameerah Al-Mustafa 《Geophysical Prospecting》2006,54(6):667-680
We present a seismic Test Line, provided by Saudi Aramco for various research teams, to highlight a few major challenges in land data processing due to near‐surface anomalies. We discuss state‐of‐the‐art methods used to compensate for shallow distortions, including single‐layer, multilayer, plus/minus, refraction and tomostatics methods. They are a starting point for the new technologies presented in other papers, all dealing with the same challenging data described here. The difficulties on the Test Line are mostly due to the assumption of vertical raypaths, inherent in classical applications of near‐surface correction statics. Even the most detailed velocity/depth model presents difficulties, due to the compleX‐raypath. There is a need for methods which are based on more complex models andtheories. 相似文献
13.
In tomographic statics seismic data processing, it is crucial to determine an optimum base for a near-surface model. In this paper, we consider near-surface model base determination as a global optimum problem. Given information from uphole shooting and the first-arrival times from a surface seismic survey, we present a near-surface velocity model construction method based on a Monte-Carlo sampling scheme using a layered equivalent medium assumption. Compared with traditional least-squares first-arrival tomography, this scheme can delineate a clearer, weathering-layer base, resulting in a better implementation of datuming correction. Examples using synthetic and field data are used to demonstrate the effectiveness of the proposed scheme. 相似文献
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Emil Blias 《Geophysical Prospecting》2013,61(3):574-581
I introduce a new explicit form of vertical seismic profile (VSP) traveltime approximation for a 2D model with non‐horizontal boundaries and anisotropic layers. The goal of the new approximation is to dramatically decrease the cost of time calculations by reducing the number of calculated rays in a complex multi‐layered anisotropic model for VSP walkaway data with many sources. This traveltime approximation extends the generalized moveout approximation proposed by Fomel and Stovas. The new equation is designed for borehole seismic geometry where the receivers are placed in a well while the sources are on the surface. For this, the time‐offset function is presented as a sum of odd and even functions. Coefficients in this approximation are determined by calculating the traveltime and its first‐ and second‐order derivatives at five specific rays. Once these coefficients are determined, the traveltimes at other rays are calculated by this approximation. Testing this new approximation on a 2D anisotropic model with dipping boundaries shows its very high accuracy for offsets three times the reflector depths. The new approximation can be used for 2D anisotropic models with tilted symmetry axes for practical VSP geometry calculations. The new explicit approximation eliminates the need of massive ray tracing in a complicated velocity model for multi‐source VSP surveys. This method is designed not for NMO correction but for replacing conventional ray tracing for time calculations. 相似文献
16.
The first-order perturbation theory is used for fast 3D computation of quasi-compressional (qP)-wave traveltimes in arbitrarily anisotropic media. For efficiency we implement the perturbation approach using a finite-difference (FD) eikonal solver. Traveltimes in the unperturbed reference medium are computed with an FD eikonal solver, while perturbed traveltimes are obtained by adding a traveltime correction to the traveltimes of the reference medium. The traveltime correction must be computed along the raypath in the reference medium. Since the raypath is not determined in FD eikonal solvers, we approximate rays by linear segments corresponding to the direction of the phase normal of plane wavefronts in each cell. An isotropic medium as a reference medium works well for weak anisotropy. Using a medium with ellipsoidal anisotropy as a background medium in the perturbation approach allows us to consider stronger anisotropy without losing computational speed. The traveltime computation in media with ellipsoidal anisotropy using an FD eikonal solver is fast and accurate. The relative error is below 0.5% for the models investigated in this study. Numerical examples show that the reference model with ellipsoidal anisotropy allows us to compute the traveltime for models with strong anisotropy with an improved accuracy compared with the isotropic reference medium. 相似文献
17.
We use controlled-source seismic interferometry (SI) and inversion in a unique way to estimate the location of near-surface scatterers and a corner diffractor by using non-physical (ghost) scattered surface and body waves. The ghosts are arrivals obtained by SI due to insufficient destructive interference in the summation process of correlated responses from a boundary of enclosing sources. Only one source at the surface is sufficient to obtain the ghost scattered wavefield. We obtain ghost scattered waves for several virtual-source locations. To determine the location of the scatterer, we invert the obtained ghost traveltimes by solving the inverse problem. We demonstrate the method using scattered surface waves. We perform finite-difference numerical simulations of a near-surface scatterer starting with a very simple model and increase the complexity by including lateral inhomogeneity. Especially for the model with lateral variations, we show the effectiveness of the method and demonstrate the estimation of the subsurface location of a corner diffractor using S-waves. In all models we obtain very good estimations of the location of the scatterer. 相似文献
18.
针对地表剧烈起伏,速度纵、横向变化大的复杂区,层析静校正较以往的折射波静校正方法有明显的优势,但是受初至时间拾取精度、炮检距的选择、近地表模型约束等问题的影响,层析反演的精度还不能满足低幅度构造预测的需求,为此提出微测井约束分步层析的静校正方法,即将浅层速度模型与最终近地表模型分步进行层析反演,并应用微测井信息约束浅层速度模型层析反演,有效地提高了近地表速度模型反演的精度,通过在古峰庄地区的应用较好地解决了复杂地表条件下的静校正问题,低幅度构造预测精度得到提高. 相似文献
19.
基于布格重力异常相对于地形起伏光滑分布的约束条件,从一维自由空气重力异常数据出发,采用贝叶斯方法估算近地表岩石密度,同时采用三次B样条函数拟合布格重力异常,获取光滑分布的布格重力异常.数据拟合和光滑约束之间的权重采用Akaike贝叶斯准则(ABIC准则)自动确定.均匀剖分模型和不均匀剖分模型数据试验都验证了该方法的有效性.相关参数评价表明,足够多的样条系数可以提高估计结果的准确性,样条系数的个数接近测点数时可获得较稳定的估计结果.增大异常的噪声水平时,ABIC准则可有效地自动增大先验光滑约束的权重.云南地区两条重力剖面应用结果表明,剖面沿线的近地表密度值起伏变化明显(达2.45~2.8g·cm^-3),前寒武纪和古生代地层密度相对较高(主要为2.53~2.75g·cm^-3),而中生代密度较低(2.45~2.73g·cm^-3);本文估计的近地表密度结果与区域物性资料及地表地质特征较吻合;估计的剖面布格重力异常具有光滑性;红河断裂两侧近地表密度差异较大,可达0.4g·cm^-3.本文获得的两条剖面近地表密度结构和布格重力异常为该区深部结构与构造研究提供更可靠的重力基础数据. 相似文献
20.
A new ray-tracing method called linear traveltime interpolation (LTI) is proposed. This method computes traveltimes and raypaths in a 2D velocity structure more rapidly and accurately than other conventional methods. The LTI method is formulated for a 2D cell model, and calculations of traveltimes and raypaths are carried out only on cell boundaries. Therefore a raypath is considered to be always straight in a cell with uniform velocity. This approach is suitable to tomography analysis. The algorithm of LTI consists of two separate steps: step 1 calculates traveltimes on all cell boundaries; step 2 traces raypaths for all pairs of receivers and the shot. A traveltime at an arbitrary point on a cell boundary is assumed to be linearly interpolated between traveltimes at the adjacent discrete points at which we calculate traveltimes. Fermat's principle is used as the criterion for choosing the correct traveltimes and raypaths from several candidates routinely. The LTI method has been compared numerically with the shooting method and the finite-difference method (FDM) of the eikonal equation. The results show that the LTI method has great advantages of high speed and high accuracy in the calculation of both traveltimes and raypaths. The LTI method can be regarded as an advanced version of the conventional FDM of the eikonal equation because the formulae of FDM are independently derived from LTI. In the process of derivation, it is shown theoretically that LTI is more accurate than FDM. Moreover in the LTI method, we can avoid the numerical instability that occurs in Vidale's method where the velocity changes abruptly. 相似文献